Lock-in amplifiers are widely used to extract weak signals from large amounts of noise. The use of a lock-in amplifier enables accurate measurements may be made even when a small signal is obscured by a thousand-fold larger noise source. Lock-in amplifiers employ a synchronous detection technique to extract a desired signal component at a specific reference frequency and phase. As a result, any noise signal components at frequencies other than the reference frequency are rejected. The output of a lock-in amplifier is typically a DC signal proportional in magnitude to a low pass filtered product of the input signal and an output of a periodic function generator.
FIG. 1 illustrates a conventional signal measurement system 1 that includes a lock-in amplifier 2. A source 3 provides a stimulus signal to a unit under test (UUT) 4 and a corresponding reference signal to a first input of the lock-in amplifier 2. In response to the stimulus signal the UUT 4 outputs an analog signal to a second input of the lock-in amplifier 2. Using the reference signal, the lock-in amplifier 2 extracts a desired signal component from the analog signal and outputs same as a measured output signal. Any noise signal components of the analog output signal, which are not correlated with the reference signal, are rejected and, ideally, do not appear within the measured output signal. It can be appreciated that the lock-in amplifier operates in a manner similar to a synchronous detector or to a bandpass filter that is tuned to the frequency of the desired signal component.
Although suitable for use in many applications, the conventional lock-in amplifier of FIG. 1 can precluded from use in those applications where, by example, a suitable reference signal input is not available, or where the output of the UUT 4 is available only in a nonreal-time digital format.